THIS!

And btw really what list :P

Others i'd recommend are naruto, GTO, Slam Dunk, 20th century boys, death note, fairy tail, reborn etc...

Seriously...

You should all be ashamed of yourselves...

Why Code Geass hasn't been mentioned by now puzzles me.

Because there are dozens of anime that should be seen before it.  You should really have a good bit of mecha under your belt before you approach the show, anyways, so you can focus on not missing one of the eleventy-seven plot twists and deus ex machina.

I'll list some anime that pretty much everyone should watch regardless of their interests:

Cowboy Bebop
Samurai Champloo
The Tatami Galaxy
Planetes
5 Centimeters Per Second
Pale Cocoon (~20 minute short)
Casshern Sins
Anything by Studio Ghibli/Miyazaki
Time of Eve
FLCL (requires watching a good bit of Gainax's previous study to completely understand)

If you're into romance even a smidge:

Eureka 7
Bokura Ga Ita
Ouran High School Host Club
ToraDora
The GIrl Who Leapt Through Time
The Place Promised in Our Early Days
Summer Wars
Welcome to the NHK

If you like action:

Full Metal Panic
Full Metal Alchemist
Black Lagoon
Kure-nai
Darker than Black
Wolf's Rain

Lmfao, so true, Code Geass holds the crown with amazing twists that you didn't remotely expect.

Ignore all the posts above me.

WATCH Hajime no Ippo!!!

That's precisely the reason there are dozens of anime that come before it. :P  The plot doesn't develop logically at all.  It's feels like you're in different worlds between the different major plot twists.

One Piece, Queen's Blade, Ah! My Goddess, and the Melancholy of Haruhi Suzimiya (the fun of that is you can skip 6 episodes of the second season and literally miss nothing)

Also Arakawa Under the Bridge and Fullmetal Alchemist Brotherhood

Psh, the plot twists are perfectly logical! Just done really really well, it all makes sense and is coherent if you pay attention.

Rurouni Kenshin is awesome ! Neo Genesis Evangelion is great too !